1. Field of the Invention
This invention relates to a gas laser apparatus which is improved in oscillation efficiency.
2. Prior Art
FIG. 69 is a perspective view showing a conventional carbon dioxide laser apparatus of the wave guide type disclosed, for example, in R. Nowack et al., "High Power CO.sub.2 Wave Guide Laser of the 1 kW Category", SPIE (Society of Photooptical Instrumentation Engineers), Vol. 1276, Proceedings, "CO.sub.2 Lasers and Application" (1990), pp. 18-28, FIG. 1. Referring to FIG. 69, reference numerals 1 and 2 each denote a discharge exciting metal electrode. Reference numeral 3 denotes an excitation power source (radio frequency power source here) connected to the metal electrode 1. Reference numerals 10 and 20 denote dielectric plates made of, for example, ceramics. The dielectric plates 10, 20 are opposite to each other and held in close contact with the metal electrodes 1 and 2, respectively. Reference numeral 4 denotes a discharge space (filled with mixture gas of CO.sub.2 --He--N.sub.2 which serves as a laser medium) defined between the dielectric plates 10 and 20. Reference numerals 5 and 6 denote arrow marks indicating the directions of coming in and going out of electrode cooling water to and from the electrodes, respectively. Reference numeral 7 denotes a total reflection mirror (resonator mirror). Reference 8 denotes output coupler (resonator mirror). Reference numeral 9 denotes a laser beam, and reference characters 21a and 21b denote inlet and outlet ports, respectively, for electrode cooling water provided at the electrode 1 (similar inlet and outlet ports (not shown) for electrode cooling water are provided also at the electrode 2).
Subsequently, the operation of the apparatus will be described. When the metal electrode 1 is connected to the RF power source 3 and the other metal electrode 2 is connected to the ground, the RF discharge for exciting the laser is caused in the discharge space 4 filled with the mixture gas described above. Thus, the discharge energy is converted into light energy by an optical resonator constituted of the total reflection mirror 7 and the output coupler 8 and is outputted as a laser beam 9 from the output coupler 8.
In a carbon dioxide gas laser, since the energy level at a low level of the laser is low, as the temperature of the gas rises, the low level concentration increases and the laser oscillation efficiency drops. Consequently, the cooling capacity of the laser gas makes a great factor which determines the laser oscillation efficiency. The ratio w/d between the major side (length w) and the minor side (gap length d) of a section of the rectangular discharge space 4 is called aspect ratio, and from the point of view of cooling of the gas serving as a laser medium, it is deduced that, when the aspect ratio is equal, the cooling capacity is similar.
In particular, when the same power is thrown in, if the aspect ratio is equal, then the temperature of the gas is equal. Accordingly, in order to throw in a high power and cool the gas sufficiently to raise the laser oscillation efficiency, the aspect ratio should be set to a high value. In addition, for laser oscillation for which a high power density is required, the minor side d should be set to a small value.
The cooling capacity for gas with respect to the length d of the minor side of the section of the rectangular discharge space 4 is shown in FIG. 70. In FIG. 70, a solid line indicates a power density at which the temperature of the gas is 250.degree. C. where the composition of the gas is He--N.sub.2 --CO.sub.2 =80--10--10 (%; rate in volume, molar fraction). It can be seen from FIG. 70 that the cooling capacity for the gas rises as the minor side d decreases.
On the other hand, when the minor side (gap length) d is set to be short, the loss a in the propagation process of the laser light increases. The propagation loss a of the EH.sub.nm mode in the rectangular wave guide can be represented by the following formula. ##EQU1## wherein .parallel. represents the laser wavelength; .epsilon. and .epsilon..sub.0 represent the permitivity with respect to the laser wavelength and the dielectric constant in vacuum (0.8854.times.10.sup.-11 CV.sup.-1 m.sup.-1); and u.sub.nm represents the coefficient with respect to the order of each mode.
FIG. 71 shows the result obtained by calculating, from the above formulae, the relationship between the gap length d and the propagation loss a where Al.sub.2 O.sub.3 (alumina) is used for a dielectric material and a wavelength (10.6 .mu.m) of a carbon dioxide laser is used as a laser wavelength. As a result, the propagation loss a increases in proportion to the gap length d.sup.-3.
The normal wave guide type carbon dioxide gas laser apparatus is often used in the range of 1.5.ltoreq.d.ltoreq.2.5 (mm) in consideration of the cooling capacity of gas and the propagation loss of light. Due to the high output, when the length of the dielectric is long, the propagation loss naturally increases. Therefore, it is necessary to increase the gap length d to provide a higher output.
FIG. 72 shows a result of an examination of the influence of the power source frequency of the RF power source 3 upon the output of the carbon dioxide gas laser in the condition of the gap length d=2 mm. It is confirmed that, as the power source frequency increases, the laser output increases dramatically. The reason is given below.
FIG. 73 shows a result of calculation of the electric field distribution in the direction of the gap d varying the frequency of the power source in the condition of the gas pressure of 80 Torr. In FIG. 73, reference character Z denotes a distance in the electric field direction, and Z=0 represents the center of the gap while Z=1.0 (mm) represents a boundary to a dielectric plate. As apparent from FIG. 73, it can be confirmed that, as the power source frequency increases, the region in which the electric field is high decreases while the low electric field region which is suitable for laser oscillation increases. Accordingly, if the power source frequency is raised, the low electric field region increases and the excitation efficiency of the laser rises as seen in FIG. 73.
This variation of the electric field distribution can be explained from a discharge maintaining mechanism. The discharge maintaining mechanism is roughly explained from the relationship between the travel time t.sub.e of electrons through the gap d and the half period t.sub.s of the power source. In particular, in such a case that electrons drifting toward the anode collide with the anode (electrode), since the number of electrons and loss of energy are high, the electric field must provide the energy which compensates for the loss. Accordingly, the high electric field region becomes wide. This corresponds to the case wherein the half period t.sub.s of the power source is longer than the gap travel time t.sub.e of electrons. On the contrary when the variation of the electric field (half period t.sub.s of the power source) is shorter than the travel time t.sub.e of electrons, the polarity of the electrode is reversed (to the negative) before electrons drifting toward the anode arrive at the anode, and consequently, the electrons are urged back and will not collide with the electrode wall. Accordingly, in this instance, the loss in the number of electrons and the energy loss are small and the high electric field region may be made narrow.
Although it may be different depending upon conditions, in the conditions calculated in connection with FIG. 72, since the drifting speed of electrons is almost 10.sup.7 cm/s, the gap travel time t.sub.e is 0.2 cm (2 mm)/10.sup.7 cm/s=2.times.10.sup.-8 sec. The critical frequency at which the time t.sub.e corresponds to one half period of the RF power source 3 is 100 MHz. Accordingly, when the frequency of the RF power source is lower than 100 MHz, the high electric field region becomes wide as shown in FIG. 73 and the lower excitation efficiency drops as shown in FIG. 72.
By the way, the conventional carbon dioxide gas laser apparatus shown in FIG. 69 employs a hybrid resonator in order to generate a laser beam of high convergency from the rectangular discharge space 4. In particular, the hybrid resonator operates, in the direction of the minor side d of the rectangular discharge space 4, as a wave guide resonator in which laser light propagates while being reflected by the dielectric plates 10 and 20, and operates, in the direction of the major side, as an unstable resonator (a resonator of the type in which light is not enclosed completely).
In the case where a wave guide is employed resonator, if the distance (L.sub.wm) between an end of a wave guide (dielectric plates 10 and 20) and a resonator mirror (reflecting mirror 7 and output coupler 8) is set to a great value, then the rate at which light escapes from the resonator becomes high, and consequently, the output efficiency of a laser beam drops. It is known that the loss by escapement of light increases in proportion to (L.sub.wm).sup.3/2. Thus, for example, in the conditions of the wave length of 10.6 .mu.m (CO.sub.2 laser) and the gap length of d=2 mm, in order to suppress the loss of light low, it is necessary to set L.sub.wm to a small value of 10 mm or so.
Accordingly, when the applied voltage is raised in order to increase the discharge power, not only the discharge occurs in the main discharge space 4, but also the discharge 41 toward the output coupler 8 occurs as seen in FIG. 74. In this instance, if the discharge occurs toward the output coupler 8, then the energy thrown in to the main discharge space 4 decreases and the laser excitation efficiency drops as seen from FIG. 75. (In FIG. 75, a point Ps denotes a discharge start power to the mirror.)
Further, if corners of the dielectric plates 10 and 20 are present in the proximity of end portions of the metal electrodes 1 and 2, then when the applied voltage rises, the electric field strengths at the corners of the dielectric plates 10 and 20 become high as seen in FIG. 76, and the discharge 42 is liable to be concentrated also at locations around the corners.
Since the conventional laser apparatus is constructed as described above, in the case where the length of the dielectric is desired to be longer in order to obtain a high output wave guide type laser, it is required to set the gap length d to be short in view of cooling whereas it is required to set the gap length d to be long in view of propagation loss of light. This is exactly a contradictory requirement, which is impossible to realize.
Further, if the permitivity .epsilon. with respect to the laser wavelength is set to be small from the above-described formulae (1) and (2), the propagation loss a is expected to be reduced. Actually, however, a material having a low permitivity is difficult to be sintered, often making it impossible to manufacture.
As will be described later, the dielectric used in this system is not only required to have a nature as a wave guide path surface but also to have a function as a capacitor for discharge such as a withstand voltage. For this reason, materials which satisfy with these conditions have been extremely restricted.
Further, in the case of the conventional CO.sub.2 laser apparatus, the optimal frequency in the laser excitation is in the vicinity of 150 MHz. However, since this frequency is limited for use thereof under the Japanese Radio-wave Law, there remains a great problem in the case of providing a general-purpose apparatus. Moreover, such an RF power source is expensive, and matching between the RF power source and a laser load is difficult. There are many problems as described.
Consequently, the conventional gas laser apparatus further has a problem that, if the applied voltage is raised in order to increase the discharge power, then not only does the discharge occur in the main discharge space but also discharge 41 toward the resonator mirror and discharge 42 concentrated at the corners of the dielectric plates occur, which deteriorates the stability of the laser apparatus.